The use of various thermoplastic resins to make fibers and fabrics is well known. Examples of such resins include polyesters, polyetheresters, polyamides and polyurethanes. Polyolefins, particularly propylene homopolymers and copolymers, are thermoplastic resins commonly used to make fibers and fabrics.
Propylene impact copolymers are a type of thermoplastic resin commonly used in applications where strength and impact resistance is desired such as in molded and extruded automobile parts, household appliances, luggage and furniture. Propylene homopolymers are often unsuitable for such applications because they are too brittle and have low impact resistance particularly at low temperature, whereas propylene impact copolymers are specifically engineered for applications such as these.
Though sometimes used to make films, propylene impact copolymers have not been used to make fibers and fabrics because impact resistance is not a required property for such applications. For fibers and fabrics, manufacturers focus on properties such as strength, processability, softness and breathability.
The use of propylene homopolymers, copolymers and various blends to make nonwoven fabrics is well known. For example, U.S. Pat. Nos. 5,460,884, 5,554,441 and 5,762,734 describe the use of polypropylene blends to prepare nonwoven fabrics. U.S. Pat. No. 5,994,482 describes the use of a polypropylene alloy for making soft nonwoven fabrics.
Though a variety of properties can be obtained, the use of blends such as these has the primary disadvantages associated with the additional processing steps required to make and use blended materials. We have discovered that many of these same properties can be obtained using a propylene impact copolymer which is not post reactor blended.
A typical propylene impact copolymer contains two phases or components, a matrix component and a copolymer rubber component dispersed in the matrix. These two components are usually produced in a sequential polymerization process wherein the homopolymer produced in one or more initial reactors is transferred to one or more subsequent reactors where copolymer is produced and incorporated within the matrix component. The copolymer component has rubbery characteristics and provides the desired impact resistance, whereas the matrix component provides overall stiffness.
Many process variables influence the resulting impact copolymer and these have been extensively studied and manipulated to obtain various desired effects. For example U.S. Pat. No. 5,166,268 describes a "cold forming" process for producing propylene impact copolymers where finished articles are fabricated at temperatures below the melting point of the preform material, in this case, the propylene impact copolymer. The patented process uses a propylene impact copolymer comprised of either a homopolymer or crystalline copolymer matrix, or first component, and at least ten percent by weight of an "interpolymer" of ethylene and a small amount of propylene (the second component). Adding comonomer to the first component lowers its stiffness. The ethylene/propylene copolymer second component is said to enable the finished, cold-formed article to better maintain its shape.
U.S. Pat. No. 5,258,464 describes propylene impact copolymers with improved resistance to "stress whitening." Stress whitening refers to the appearance of white spots at points of impact or other stress. These otherwise conventional propylene impact copolymers have first and second components characterized by a numerical ratio of the second component intrinsic viscosity to the first component intrinsic viscosity which is near unity.
In U.S. Pat. No. 5,362,782, nucleating agent is added to propylene impact copolymers having a numerical ratio of the intrinsic viscosity of the copolymer rubber phase (second component) to the intrinsic viscosity of the homopolymer phase (first component) which is near unity and an ethylene content of the copolymer phase in the range of 38% to 60% by weight. These propylene impact copolymers are described as producing articles having good clarity as well as impact strength and resistance to stress whitening. The nucleating agents increase stiffness and impact strength.
Propylene impact copolymers are also used to produce films as described in U.S. Pat. No. 5,948,839. The impact copolymer described in this patent contain a conventional first component and 25 to 45 weight percent ethylene/propylene second component having from 55 to 65 weight percent ethylene.
We have discovered that fibers and fabrics prepared with impact copolymers have distinct advantages, particularly over similar products prepared with homopolymers and random copolymers.